Conventionally, aluminum, glass, ceramic, and the like are used as a substrate material for a magnetic recording medium. At present, aluminum and glass are mainly practically used depending upon sizes and uses. Of these, the use of a glass substrate is expanding year after year since it is almost free of a surface defect and is excellent in surface flatness and smoothness and surface hardness. As a glass for use as a substrate for a magnetic recording medium, a glass chemically strengthened by ion exchange and a crystallized glass are known. As a chemically strengthened glass, for example, JP-A-1-239036 discloses a chemically strengthened glass substrate having, in the vicinity of the glass surface, a compression stress layer formed by subjecting a glass containing, by weight %, 50 to 85% of SiO2, 0.5 to 14% of Al2O3, 10 to 32% of R2O (in which R is an alkali metal ion), 1 to 15% of ZnO and 1.1 to 14% of B2O3 to ion-exchange with an alkali ion.
Further, as a crystallized glass, for example, U.S. Pat. No. 5,391,622 discloses a crystallized-glass substrate for a magnetic disk, which glass substrate contains, by weight %, 65 to 83% of SiO2, 8 to 13% of Li2O, 0 to 7% of K2O, 0.5 to 5% of MgO, 0 to 5% of ZnO, 0 to 5% of PbO, the content of MgO+ZnO+PbO being 0.5 to 5%, 1 to 4% of P2O5, 0 to 7% of Al2O3 and 0 to 2% of As2O3+Sb2O3 and contains fine Li2O.2SiO2 crystal grains as a main crystal.
In recent years, however, with developments in higher-density recording, information recording devices such as a magnetic disk typified by a disk of a hard disk drive are required to cope with a conversion from a longitudinal recording mode to a perpendicular recording mode. The problem pointed out with regard to the longitudinal recording mode is as follows. A magnetic domain is easily inverted due to a heat such as the heat of a room temperature. With an increase in density, writing is no longer possible, and written data is easily lost. The above phenomenon is known as a heat fluctuation problem, and is coming to be a bar against the longitudinal recording mode. For coping with the heat fluctuation problem in the longitudinal recording mode, studies are being actively made in recent years for applying a perpendicular recording mode to practical use.
The film constitution that is known with regard to the above perpendicular magnetic recording mode includes a single-layered film in which a perpendicular magnetic recording layer is formed on a non-magnetic substrate, a two-layered film in which a soft magnetic layer and a magnetic recording layer are consecutively stacked and a three-layered film in which a hard magnetic layer, a soft magnetic layer and a magnetic recording layer are consecutively stacked. Of these, the two-layered film and the three-layered film are suitable for attaining high-density recording and maintaining the stability of a magnetic moment, so that developments are being actively made for their practical application. For improving the properties of a perpendicular magnetic recording medium of the above multi-layered magnetic film, it is said that it is required to form the film with a high-temperature sputtering apparatus or to heat-treat the formed film at a high temperature.
However, an aluminum substrate that has been generally used so far has a low heat resistance of 280° C. or lower. Further, when a conventional chemically strengthened glass is used at a temperature in a chemical strengthening temperature range (350-420° C.) or a temperature higher than such a range, a surface stress caused by ion exchange for the chemical strengthening is relaxed, and the substrate strength sharply decreases, so that it is generally required to use a chemically strengthened glass at a temperature of 350° C. or lower. In the perpendicular magnetic recording mode, it is required to form a film with a sputtering machine at a high temperature of 400° C. or higher or anneal the film of stacked magnetic layers at a temperature of 400° C. or higher, for improving a magnetic film in magnetic characteristic. It is therefore clear that none of aluminum substrates and chemically strengthened glass substrates commercially available at present can cope with the above requirement, and a glass substrate having high heat resistance is demanded.
For attaining excellent heat resistance, a glass material that does not contain any alkali metal oxide so far essential for a chemically strengthened glass may be taken into account. In this case, however, there is caused a problem that the meltability of such a glass decreases since the glass viscosity during melting increases. Further, when a magnetic disk is incorporated into a drive unit, it is required to impart a glass substrate with a larger thermal expansion coefficient for conforming the thermal expansion coefficient of the glass substrate to the counterpart of a metal fixing member to fix a disk. However, it is difficult to impart a glass containing no alkali metal oxide with the thermal expansion coefficient that a substrate for an information recording medium is required to have.
For attaining a high recording density of an information recording medium, the employment of the above perpendicular magnetic recording mode only is not sufficient, and how closely a read/write head can be brought close to the medium surface is also another problem to solve. The distance between the above head and the medium surface is called a flying height, and the flying height can be decreased only when a high surface flatness and smoothness can be attained for a substrate as a base for an information recording medium and when a substance adhering to the substrate surface can be reduced.
For a substrate made of a glass for an information recording medium, a glass is processed into a substrate having a flat and smooth surface by polishing and lapping. However, polishing grains used for the polishing and fine glass dust scraped from the glass are adhering to the surface of the thus-processed glass substrate. Therefore, the processed substrate is immersed and cleaned in an acid thereby to remove a surface soiling and a very thin layer of the surface by etching.
However, when a glass has low durability against the acid used for the cleaning, the acid roughens the surface of a substrate made of the glass, and the high surface flatness and smoothness obtained by the polishing are inevitably impaired in return for the surface cleanness.
That is, for attaining a high recording density of an information recording medium, it is required to provide a glass material that not only permits bringing of a substrate surface into a clean state by cleaning it with an acid but also prevents the above cleaning from roughening a smooth surface, and which is not deformed by the high-temperature treatment.